The genetic and molecular mechanisms controlling the regular and precise segregation of chromosomes in mitosis and meiosis are poorly understood. This research is directed toward a genetic characterization of aneuploid mitotic chromosome behavior in yeast (Saccharomyces cerevisiae), a simple eukaryote that may be easily manipulated genetically. Two coordinate approaches are being employed. (1) We are attempting to isolate and characterize the complete set of single disomic haploids in yeast. The mitotic stability of each disome will be determined, and relationships among individual aneuploid chromosomes in fostering mitotic stability or instability will be explored. (2) The disomes isolated in this study will be employed as genetic "reagents" for the isolation and characterization of mutations conditionally defective in mitotic segregational control.